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Old 2nd August 2012, 09:07 PM   #1
Corpius is offline Corpius  Netherlands
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Default A circuit to monitor the analog audio line level

Hello,

I'm searching for a good circuit to monitor the analog audio line level inside my power amps. I have a arduino lying around and want to use its adc to measure the signal strength. If there is no signal for 15 minutes or so the Arduino can switch a power relay to switch off the amps.

The circuit may not degrade the audio signal. I have been looking around, but can't find much information on this.

Any help like schematics, links, or else would be appreciated.
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Old 2nd August 2012, 09:50 PM   #2
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Should be fairly straightforward. Bias the ADC input at half supply with e.g. 100k + 100k in series. Then put a 1uF cap between the junction of the resistors and the input socket of your amp.

The Arduino will need its own power supply/transformer. Connect the 0V of the CPU supply to the signal gnd of your power amp, i.e. the gnd connection of the input socket.
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Old 2nd August 2012, 10:17 PM   #3
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Signal Detecting Auto Power-On Unit

a guy from some Dutch forum gave me this link. It is excactly what I was looking for and no arduino is needed anymore, meaning that I can use it for another project.

Thanks anyway!
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Old 4th August 2012, 07:31 AM   #4
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Quote:
Originally Posted by Corpius View Post
Signal Detecting Auto Power-On Unit

a guy from some Dutch forum gave me this link. It is excactly what I was looking for and no arduino is needed anymore, meaning that I can use it for another project.

Thanks anyway!
Thats very similar to a circuit I designed many years ago.

As audio performance is unimportant I used micropower opamps (TL062) and very high impedances throughout. The circuit has a 4.8v nicad that enables it to detect audio without any mains power to the unit itself (in standby). The output of the trigger circuit drives a 6 pin DIL optotriac which then drives a mains relay. The Nicad pack is recharged when in use.
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Old 5th August 2012, 08:35 AM   #5
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Quote:
Originally Posted by Mooly View Post
Thats very similar to a circuit I designed many years ago.

As audio performance is unimportant I used micropower opamps (TL062) and very high impedances throughout. The circuit has a 4.8v nicad that enables it to detect audio without any mains power to the unit itself (in standby). The output of the trigger circuit drives a 6 pin DIL optotriac which then drives a mains relay. The Nicad pack is recharged when in use.

I like the idea of using a nicad accu. You are probably using a simly power supply (accu charger) to charge the nicad, right? If so, then the accu charger probably is also taking over powering the relay. Or am I wrong?
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Old 5th August 2012, 11:24 AM   #6
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Would it be better to place the `input resistors` (R1 &R2) as close as posible to the actual line inputs?

Signal Detecting Auto Power-On Unit
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Old 5th August 2012, 12:15 PM   #7
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Yes, I've used the same idea a couple of times. One was in a headphone amp that detects audio and then powers up,
Advice for hearing impaired setup on a new television?

And the other uses an opto (as in just light down a fibre optic TOSLINK lead) to detect when the main amp is on and it then powers a relay that powers up all the source components. That was a separate small wall mounted box.

In the headphone amp the nicad is charged from the main PSU when it's on.

Rods design has the inputs connected via 10K's and I suppose technically (depending on the source component) it could be argued that for ultimate fidelity and quality that's not ideal.

I seem to remember using a virtual ground input stage and higher value resistors to reduce loading (although used on a TV quality wasn't a prime issue).
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Old 5th August 2012, 03:05 PM   #8
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Originally Posted by Mooly View Post
Rods design has the inputs connected via 10K's and I suppose technically (depending on the source component) it could be argued that for ultimate fidelity and quality that's not ideal..
For now I'm using a "DAC --> pre-amp --> power amp" configuration. My DAC can also be used as pre-amp, but I have not compared the yet. If it turns out that it sounds better, my current pre-amp will be left out.

So basically the signal is and stays a analog line signal. what would you recommend instead of the 10K resistors?
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Old 5th August 2012, 06:57 PM   #9
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OK, a couple of ideas then.

If using Rods circuit as it is, then I would first build it and confirm it works OK.

Then you could replace the opamp with FET type (to get rid of DC offset issues) You'll see why in a minute.

Increasing R1 and R2 to 100K or even 1Meg. R3 would be raised in proportion to 1meg or 10 meg. That would reduce the loading on the source to pretty much negligable. C1 could be lowered to 10nf or less.

Remember the circuit only has to detect audio. The frequency response and squarewave response is immaterial.

Another option would be to reconfigure the first opamp as a mixer type inverting stage. The two inputs L and R would be say 1meg and the feedback resistor 10meg. Pin 3 would be connected to the 6 volt line.
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